The development of stable and non-toxic near-infrared optical probes together with the recent advances in laser diode sources and detection technologies promise non-invasive, diagnostic tissue spectroscopy. However, studies show that determination of biochemical species from re-emitted tissue fluorescence spectra require deconvolution of tissue scattering and absorption properties for quantitative tissue spectroscopy. Since tissue optical properties can be expected to vary from person to person and with pathophysiology, it is unlikely that fluorescence intensity measurements will make full use of the emerging NIR optical probes. Signals which arise from endogenous species such as NAD(P)H, NAD(P), and FP are likewise influenced by tissue optical properties. In this application, the applicants proposed to couple established techniques of frequency-domain measurements of optical probe lifetimes together with a simple algorithm for excitation and fluorescent light propagation in tissues in order to quantitate metabolite concentrations using exogenous probes and metabolic state, using endogenous probes. Both computations and experimental measurements demonstrate the feasibility of our approach for lifetime-based tissue spectroscopy, in which the kinetics of fluorescent re-emission are decoupled from photon "time-of-flights" in order to provide diagnostic measurements from endogenous and exogenous fluorescent probes. The applicants proposed to conduct measurements which validate (i) fluorescence-lifetime tissue spectroscopy in which a uniform distribution of probe lifetime exists, (ii) fluorescence-lifetime diagnostics in which an exogenous, metabolite-sensing fluorescent probe is immobilized in an implanted device, and (iii) fluorescence lifetime imaging in which a "map" or image of lifetime provides a metabolic "map" or image of diseased tissues from non-invasive optical measurements made at the tissue-air interface. These fundamental studies are pointed toward applications for biodiagnostic sensing and imaging.